1. Field of the Invention
The present invention relates to a process of forming an electrode-to-electrode bond structure. More specifically, the present invention relates to a process of forming an electrode-to-electrode bond structure which can be applied to e.g. bonding as well as electrically connecting a semiconductor chip to another semiconductor chip, mounting a semiconductor chip on a wiring board, and connecting a wiring board to another wiring board.
2. Description of Related Art
There is a growing demand in recent years for increased density in mounting of electronic parts on e.g. a printed wiring board and a ceramic substrate. As a way for satisfying such a demand, a bear-chip mounting method is attracting attention. In the bear-chip mounting method, conventional face-up mounting is being taken over by a face-down mounting, i.e. flip chip bonding. In the face-up mounting, electric connection between the semiconductor chip and the wiring board is established usually by means of wire bonding, whereas in the face-down mounting, electrical connection is established by solder bumps between the semiconductor chip and the wiring board. This technique of establishing electrical connection via the solder bumps or solder material is also applied to a connection between two separate semiconductor chips or between two separate wiring boards, as disclosed in JP-A-2-96343, JP-A-4-326747, JP-A-5-326628, JP-A-6-262386, JP-A-8-64639, JP-A-9-260059, JP-A-11-135552, JP-A-11-191673 for example.
FIGS. 6a through 6j show a conventional method for making a flip chip bonding. According to the conventional flip chip bonding method, first, as shown FIG. 6a, a metal mask 430 is prepared, in which openings 430a are formed at positions corresponding to electrodes 411 of a semiconductor 410.
Next, as shown in FIG. 6b, the metal mask 430 is placed on the semiconductor chip 410 with the openings 430a aligned with the correspondingelectrodes 411.
Next, as shown in FIG. 6c, a solder paste 440 containing a predetermined solder powder is filled into the openings 430a by means of printing.
Next, as shown in FIG. 6d, the metal mask 430 is removed from the surface of the semiconductor chip 410, leaving the solder paste 440.
Next, as shown in FIG. 6e, a heating step follows for melting the solder powder in the solder paste 440 to form bumps 412 on the electrodes 411.
After the formation of the bumps 412 on the electrodes 411 of the semiconductor chip 410, a flux 450 is applied on the wiring board 420, as shown in FIG. 6f. The flux 450 serves to remove an oxide coating from the surface of the bumps 412 while preventing the bumps 412 from re-oxidizing by prohibiting contact with air during the subsequent re-flow soldering step. The flux 450 also performs an additional function of providing preliminary fixation of the semiconductor chip 410 onto the wiring board 420.
Next, as shown in FIG. 6g, the semiconductor chip 410 is placed on the wiring board 420 with electrodes 421 of the wiring board 320 aligned with the corresponding bumps 412.
Next, as shown in FIG. 6h, a heating step for re-flowing the bumps 412 follows to connect the electrodes 411 and the electrodes 421 with the bumps 412.
Next, as shown in FIG. 6i, the flux 450 is washed and removed. In this way, the flip chip bonding of the semiconductor chip 410 to the wiring board 420 is established.
Finally, as shown in FIG. 6j, an adhesive or an under-fill resin 460 is loaded between the semiconductor chip 410 and the wiring board 420. The under-fill resin 460 protects the bump 412 that serves as a conductor to connect the electrode 411 and the electrode 421 while also protecting the surface of the semiconductor chip 410 and the surface of the wiring board 420, thereby maintaining the bond reliability for a long time.
However, according to the conventional bonding process described above, when the metal mask 430 is placed on the semiconductor chip 410, the openings 430a must be aligned with the electrodes 411, which becomes increasingly difficult as the electrodes 411 are disposed at a smaller pitch. In particular, when the electrodes 411 are disposed at a pitch of not greater than 200 μm, the relative magnitude of an alignment error in placing the metal mask 430 becomes very large. Thus, the alignment error in the metal mask 430 results in positional error of the bumps 412 and may cause damage or loss of electric conduction in the flip chip bonding.
When the electrodes 411 are disposed at a pitch not greater than 200 μm, and if the size of electrodes 412 is half the pitch, the bumps 412 formable on the electrode 411 can have a diameter of about 70 μm. After bonding via the bumps 412 of such a size, the semiconductor chip 410 and the wiring board 420 is spaced by a distance not greater than 50 μm. If the distance between the semiconductor chip 410 and the wiring board 420 is so small as such, it is difficult to remove the flux sufficiently in the process step of FIG. 6i. The flux remaining between the semiconductor chip 410 and the wiring board 420 can cause such problems as corrosion of the bumps 412, decrease of dielectric resistance between the electrodes, and insufficient filling of the under-fill resin 460. In addition, if the distance between the semiconductor chip 410 and the wiring board 420 is that small, voids can easily develop in the under-fill resin 460 in the process step of FIG. 6j, making it difficult to properly fill the under-fill resin 460 between the semiconductor chip 410 and the wiring board 420.
Thus, according to the conventional method, it is difficult to obtain a high bond reliability when the electrodes are disposed at a small pitch or at a high density.
Further, according to the above-described conventional method, a large number of steps including application and removal of the flux 450 and filling of the under-fill resin 460 must be performed. In other words, the process is complex.
For the purpose of simplifying the bonding process, a fluxing under-fill resin is used in recent years. The fluxing under-fill resin is an epoxy resin containing a flux as an additive, and is intended to serve as an under-fill resin as well as a flux. For example, the fluxing under-fill resin is applied on the wiring board 420 in the step of FIG. 6f, just as the flux is applied, and then heated, without being washed or removed, to harden between the semiconductor chip 410 and the wiring board 420 in the step of FIG. 6j, just like an ordinary under-fill resin 460.
The fluxing under-fill resin has to contain an inorganic filler in order to reduce its thermal expansion coefficient, thereby attaining reliability of the bond between the semiconductor chip 410 and the wiring board 420. However, if the inorganic filler is contained in the fluxing under-fill resin at a proportion of no lower than 20 wt %, such a large amount of the inorganic filler causes the fluxing under-fill resin to easily enter the boundary between each bump 412 and a corresponding electrode 421, resulting in a very sharp decrease of adhesion of the bump 412 relative to the electrode 421. For this reason, the addition of the inorganic filler to the extent of reducing the thermal expansion of the fluxing under-fill resin to a necessary level can result in an initial conduction failure caused by the poor bonding rate of the bumps. Another problem is that the fluxing under-fill resin is poor in utility because it is a single-liquid adhesive and has a short service life at room temperature.